Don`t get me wrong, The Tom Tom is a very easy to use unit, and its user interface is stunning. I also sell nearly 50 Tom Toms to every professional In Car Sat Nav, so from that point of view also they are good! I sell virtually all makes from Garmin, Marrelli, and a lot of the independants as well as being an Authourised Installer for practically all makes of the In Car ones also.
But . . . And its a VERY big But . . . .
The mapping is on the Tom Tom / Hand held units is suited to hikers and Bikers and designed for anything smaller than a car that can travel down green lanes and small roads with no regard for obstacles and rights of passage. The car systems give a different sort of priority to the routes they calculate to take into consideration the vehicles being used.
The In Car systems also do not track the route via satellite, but instead use a gyroscope and speed sensor to calculate the position of the vehicle on a pre-loaded Map, and only use the Satellite to confirm the position on the map every thirty seconds or so - unlike the hand held units that have no back up when the sattelites are out of sight, tunnels, tall buildings etc etc.
The car systems do have their flaws also, apart from the price - they are not as easy to input destination data as many of the cheapie Tom Toms are, and mapping upgrades are usually near £100 a time.
In my professional opinion, the ultimate system does not yet exist in the UK, but one day soon, somebody like Blaupunkt, Pioneer, Kenwood or Clarion will take a few minutes out to nick the user interface from the Tom Tom or similar and only then will you end up with a reliable and useable In Car sat nav.